I was reading a little tidbit in a parenting magazine that said 4 years between children is most ideal as a age gap because they're less affected by the time spent with a new baby. It also mentioned that anything under a 2 year difference is simply survival (so basically crazy I guess). Another article online said the best gaps are under 1 year or over 4 years because the chance of sibling rivalry is the least. I also read that because those first years are so critical for a child's development that the gap should be more than 2 years.
My kids are 20 months apart and I completely agree that it's been all about survival. Just making sure diapers are changed, everybody is fed, basic shopping, lots of playing and keeping everyone happy. The first few months were a big adjustment but once you understand the new baby and everyone is on some kind of routine it got easier. Now my kids are 9 months and 29 months and the sibling rivalry has started! They're fighting over toys and my toddler is getting really jealous of all the time I have to spend with the baby so he's having meltdowns and tantrums and screaming fits! I try as much as I can to have one on one time with each child and getting help from the grandmas when hubby isn't around but there are some really difficult days with one kid napping and one not and they eat meals at different times (I hope to change that soon)! Thankfully they both sleep great at night (most nights) so I'm well rested now and ready to take on most challenges.
So, what is the ideal gap? I think it really depends on your family situation. I didn't have my first child until I was 33 so I didn't want to wait too long to have the second because age wasn't on my side and you never know how long it'll take. Thankfully it didn't take long! If you have the first child when you're young then I think it would be ideal to wait until they were feeding themselves, potty trained and just more self sufficient before having another baby. Having 2 in diapers, helping 2 kids to eat and pretty much do everything isn't easy! Some people even opt for the HUGE gap (some I know have them 16 years apart) which I think is almost as crazy as having them really close!